Gyros measure rotation rates or changes in angular velocity about an axis. A basic conventional fiber optic gyro (FOG) includes a light source, a beam generating device, and a coil of optical fiber coupled to the beam generating device that encircles an area. The beam generating device transmits light beams into the coil that propagate in a clockwise (CW) direction and a counter-clockwise (CCW) direction along the core of the optical fiber. The two counter-propagating (CW and CCW) beams experience different path lengths while propagating around a rotating path, and the difference in the two path lengths is proportional to the rotational rate. FOGs have accuracies that generally increase with the area encircled by the optical path of the light beams. Thus, the larger the area enclosed by the optical path, the greater the signal-to-noise ratio of the FOG. Also, to improve the signal-to-noise ratio of the FOG, the optical path may be increased by increasing the number of turns of the coil.
In a resonator fiber optic gyro (RFOG), the counter-propagating light beams are monochromatic and recirculate through multiple turns of the coil and for multiple passes through the coil using a recirculator such as a fiber coupler or other reflective device. The beam generating device typically modulates and/or shifts the frequencies of each of the counter-propagating light beams so that the resonance frequencies of the resonant coil may be observed. The resonance frequencies for each of the CW and CCW paths through the coil are based on a constructive interference of successively recirculated beams in each optical path. A rotation of the coil produces a shift in the respective resonance frequencies of the resonant coil and the frequency difference associated with tuning the CW beam and CCW beam frequencies to match the coil's resonance frequency shift due to rotation indicates the rotation rate. A reflective mirror may be used to recirculate the counter-propagating light beams in the coil but this typically reduces the signal-to-noise ratio from losses generated at the transition from the mirror to the coil.
Accordingly, it is desirable to provide a fiber optic gyro capable of measuring rotational rates with an accuracy sufficient for navigation systems. In addition, it is desirable to provide a high accuracy fiber optic gyro for integration with relatively small platforms and made relatively inexpensively. Good performance of the RFOG is premised on having a low fiber-to-fiber coupling loss so that the light makes many trips through the fiber coil. The prior art in this field uses a highly reflective mirror (e.g.: 98% reflectivity) to do the fiber-to-fiber coupling. While this architecture uses the advantage that reflective mirror coatings can be made very precisely with multiple dielectric coatings, it suffers a serious disadvantage, namely that it is difficult to insure the two fiber ends are aligned to each other. An implementation of this design would require time consuming and expensive active and by-hand alignments.
FIG. 1 is a block diagram of an exemplary resonator fiber optic gyro 102. The operation of an exemplary resonator fiber optic gyro 102 is described hereinbelow, and in greater detail in the commonly assigned U.S. application having Ser. No. 11/298,439, filed on Dec. 9, 2005, and now published as U.S. 2007/0133003, which is incorporated by reference herein in its entirety. The resonator fiber optic gyro 102 comprises two lasers 104, 106 (e.g., light sources such as tunable lasers, laser diodes, or other suitable light sources) that synthesize light beams, respectively, a resonator 108 circulating light beams in counter-propagating directions and having a recirculator 110 that introduces a portion of the light beams from the lasers 104, 106 into the resonator 108, photodetectors 112, 114 that sample light circulating in the resonator 108, resonance detectors 116, 118 coupled to the photodetectors 114, 112, respectively, that detect the centers of resonance dips for each of the counter-propagating directions of the resonator 108, and servos 120, 122 having an input coupled to the resonance detectors 116, 118, respectively, and an output coupled to the lasers 104, 106, respectively. These components of the resonator fiber optic gyro 102 thus form resonance tracking loops 124, 126 for each counter-propagating direction [e.g., clockwise (CW) and counter clockwise (CCW)].
The resonator 108 comprises the recirculator 110 and optical fiber coil 136 with a plurality of loops. In an exemplary embodiment, optical fiber coil 136 is a hollow core optical fiber, although any suitable optical fiber may be used. Fiber optic coil 136 has a first end 138 and a second end 140. Each end 138, 140 is physically coupled a respective support portion of the recirculator 110. The recirculator 110 introduces CW and CCW input light beams into the optical fiber coil 136 and circulates a portion of the modulated light beams through the optical fiber coil 136. The recirculator 110 reintroduces light emerging from one end of the optical fiber coil 136 into the other end of the fiber coil 136, thus causing light to propagate through the fiber coil 136 many times. By application of the Sagnac Effect, the fiber optic gyro 102 senses a rotation rate about an axis of the fiber optic gyro 102. Efficient light recirculation requires precise alignment of the optical fiber coil ends 138, 140.